In solar cell design, it is important that an anti-reflective layer be applied to the solar cell to increase its current output and efficiency. When the surface of the solar cell is flat, it is relatively easy to uniformly deposit a layer of anti-reflective material over a large surface area. Various techniques, such as spin-on of solutions, chemical vapor deposition (CVD) and sputter deposition, have been used to deposit a layer on flat surfaces.
Recently, solar cells have been built by incorporating a plurality of silicon spheres to form a solar array which efficiently produces electricity from the sun's radiation as described in U.S. Pat. No. 4,691,076 to Levine et al., incorporated by reference herein. During production of the array, each silicon sphere is embedded in aluminum foil such that only a hemispherical surface is exposed. Unfortunately, none of the prior layering techniques produced the desired thin, uniform layer on the hemispherical surfaces of the silicon spheres used in these new solar cells.
When performing the spin-on technique, a titanium containing solution is spun onto a flat surface to form a thin layer of titanium dioxide to serve as the anti-reflective layer. However, when depositing a layer on a curved surface using the spin-on technique uniform coverage and thickness on the hemispherical surfaces of the silicon cells is difficult to obtain.
During sputtering deposition, a titanium metal is sputtered onto the desired surface and then oxidized to form titanium dioxide. However, oxidation of the metal requires high temperatures which may damage or destroy the material of the solar array.
Depositing layers by CVD methods on spherical surfaces also does not result in uniform thickness and coverage when applied to irregular or curved surfaces, such as the hemispherical surfaces of the spherical solar cells. Such deposition is anisotropic such that only those areas of the solar cell surface which are approximately perpendicular to the flow of the reacting chemicals obtain an adequate layer. Other areas of the surface, such as those on the sides of the silicon spheres, are approximately parallel to the flow of the reacting chemicals and obtain an inadequate deposit.
Therefore, a need has arisen to provide an apparatus and method for depositing a uniform layer of a titanium dioxide compound onto silicon spheres used in solar cells.